METABOLISM. 



361 



When this stage is reached, the output of nitrogen in the urine shows 

 a sudden rise for a day or two, being followed by a rapid fall in the 

 excretion of nitrogen and by the death of the animal. 



The body loses weight, the loss falling most heavily on the less 

 vital organs such as the muscles, whereas the heart and central nervous 

 system lose little or no weight. The breaking down of protein during 

 starvation is probably brought about by a process of autolysis or self- 

 digestion in the tissues ; the amino-acids formed by the disintegra- 

 tion of the less important tissues are carried in the blood stream and 

 made use of by the vital organs. A similar process of autolysis has 

 been observed in the salmon ; during its stay in fresh water the salmon 

 takes no food, and the development of the sexual organs, which takes 

 place during this period, is effected at the expense of the skeletal 

 muscles, which undergo autolysis. In man the character of the meta- 

 bolism of fat and protein is modified during starvation, as is shown by 

 the appearance of creatine and of /3-oxybutyric acid in the urine. 



The daily excretion of nitrogen in the urine during starvation 

 amounts in man to 10-12 grams, and it might be expected that if 

 this amount of nitrogen were taken in the form of protein food, it would 

 be used in replacing the daily disintegration of tissue in the body, and 

 would not appear in the urine. Experiment shows, however, that in 

 this case almost the whole of the nitrogen taken in the protein meal 

 appears in the urine in addition to that which was previously being 

 excreted, so that the disintegration of protein is still going on. When 

 protein is the sole article of diet, it is necessary, in fact, to give by the 

 mouth a quantity of protein containing 3 to 5 times as much nitrogen as 

 that which was previously being excreted during starvation in order to 

 obtain a balance between the intake and output of nitrogen. This 

 balance is called nitrogenous equilibrium. A further increase in the 

 amount of protein taken by the mouth does not lead to a retention of 

 nitrogen in the body, but the amount of nitrogen excreted increases until 

 nitrogenous equilibrium is again reached at a higher level than before. 



